1. Field of the Invention
The present invention relates to the identification of compounds that act, at physiologically-compatible levels, to inhibit the formation of Alzheimer senile amyloid plaques by inhibiting the genesis and/or growth of dense microspheres (DMS). More specifically, the present invention relates to pharmaceutically active agents that impede formation of Alzheimer senile amyloid plaques in vivo, and to a method for the screening of compounds which possess this activity.
2. Description of Related Art
There is no effective therapy for cerebral amyloidosis, (the accepted cause of Alzheimer's disease), which almost invariably has a fatal outcome following the onset of amyloid deposits. For example, Alzheimer's disease is estimated to be the fourth or fifth leading cause of death in North Americans.
A universally accepted indicator of cerebral amyloidosis is the accumulation of large numbers of lesions, so-called “senile plaques,” that are comprised in large part of amyloid fibrils. Senile plaques are spherical, ranging from 10 to 200 μm in diameter, that are found occasionally in aged adult cerebral cortex but are present in large numbers in Alzheimer-affected cortex.
The utilizing of materials found in human brain (normal or Alzheimer-affected) that are not already amyloid, and of transforming them into amyloid, has been documented in the literature. There was also description in the art of an experimental system, derived exclusively from human materials, that was characterized by the clinical manifestations of Alzheimer's disease. Because the presence of amyloid is the most qualitatively and quantitatively specific indication of senile-plaque formation, most specialists agree that reproduction of amyloid fibrils experimentally from precursor materials which are extracted, activated, or otherwise derived from human brain constitutes the best available evidence linking an agent or precursor to the progression of cerebral amyloidosis.
With the recognized importance of an experimental system that would permit testing for such a linkage, it has been possible to reproduce amyloid experimentally from materials derived solely from human brain tissue. Accordingly, reliable indicators are available for compounds that might be effective in treating cerebral amyloidosis; and it is possible to determine whether a group of compounds exists that block the conversion of a brain-localized precursor to cerebral amyloid (i.e., that display “anti-amyloid activity”) at physiologically acceptable levels of the active agent.
A microscopic structure referred to as the dense microsphere is known to exist both in normal brain and in brain affected by Alzheimer's disease. See Averback, Acta Neuropathol. 61: 148-52 (1983); results confirmed by Hara, J. Neuropath. Exp. Neurol. (1986). Evidence for the existence of dense microspheres (DMS) comes from microscopic histological section studies of fixed whole brain tissue, where the dense microspheres are seen to have a proteinaceous central region (“DMS protein”) surrounded by continuous membrane (“DMS membrane”). The dense microspheres are observed as randomly dispersed, very infrequent structures which occupy an estimated 10−9 or less of total brain volume, at a unit frequency roughly estimated at 10−14 or less relative to other definable brain structures such as mitochondria.
The extraction, purification, and characterization of isolated samples of DMS and the use of DMS material have been documented. See, for example, Averback, U.S. Pat. Nos. 4,919,915 and 4,816,416, the disclosures of which are incorporated by reference herein in their entirety.
It is well known that in the majority of cases, subjects with Alzheimer's disease and/or cerebral amyloidosis in general display symptoms on a quantitative basis Glessed, G, et al., “The association between quantitative measures of dementia and of senile change in the cerebral grey matter of elderly subjects,” British Journal of Psychiatry, 114, pp 797-811 (1968); Tomlinson, B. E., et al., “Observations on the brains of non-demented old people,” Journal of the Neurological Sciences, 7, pp 331-56 (1968); Tomlinson, B. E., et al., “Observations on the brains of demented old people,” Journal of the Neurological Sciences, 11 pp 205-42 (1970); Corsellis, J. A. N., Mental Illness and the Ageing Brain” Oxford University Press, London (1962); Corsellis, J. A. N., “Ageing and the Dementias,” Greenfield's Neuropathology, Edward Arnold, London, pp 796-848 (1976). Elderly subjects who have a small number of senile plaques are asymptomatic and are categorized by some experts as preclinical or by others as presymptomatic, or by still other authorities as normal variants. Thus, it is a common and perhaps normal feature of the elderly brain to have a low number of senile plaques, classified usually as within normal limits. However, when the amyloid plaque number is high, symptoms of dementia appear (see, references cited above). Therefore, a treatment mechanism is both useful and novel if it results in an individual at risk shifting from a high quantity senile cerebral amyloid plaque group to a low quantity senile cerebral amyloid plaque group.
DMS disruption does not commence before individual DMS reach a threshold size in the elderly or Alzheimer group. Averback, Acta Nruropathol. 61: 148-52 (1983). Therefore, a reduction in intact DMS size will delay the onset of DMS disruption, to the extent that the threshold size is not attained. There is therefore a need to provide methods for preventing DMS genesis and growth, which will prevent DMS enlargement and therefore significantly inhibit the autocatalytic phenomenon by the volume recruitment mechanism as described above.
There also is a need to provide methods that will also delay or prevent the initiation of the process which requires a minimal DMS size. Therefore, if DMS do not form at all, the particular cerebral amyloid formation which is induced by DMS disruption is totally prevented. If DMS size can be curtailed to an important extent (such as around 25% or more, for example), the DMS do not attain the threshold size for disruption, and therefore cerebral amyloid formation induced by DMS disruption can also be prevented.